Multi-color light emitting devices with compositionally graded cladding group III-nitride layers grown on substrates

ABSTRACT

A light emitting device includes a substrate, multiple n-type layers, and multiple p-type layers. The n-type layers and the p-type layers each include a group III nitride alloy. At least one of the n-type layers is a compositionally graded n-type group III nitride, and at least one of the p-type layers is a compositionally graded p-type group III nitride. A first ohmic contact for injecting current is formed on the substrate, and a second ohmic contact is formed on a surface of at least one of the p-type layers. Utilizing the disclosed structure and methods, a device capable of emitting light over a wide spectrum may be made without the use of phosphor materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Pat. ApplicationSer. No. 61/505,954, filed on Jul. 8, 2011, entitled “Multi-Color LightEmitting Devices with Compositionally Graded Cladding Group III-NitrideLayers Grown on Silicon Substrates,” which is incorporated by referenceherein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosure relates to a light emitting device (LED) based on groupIII-nitride semiconductors grown on Si substrates capable of emittinglight over a wide spectrum, e.g., from ultra-violet to near-infrared.

2. Background Discussion

Light emitting diodes (LEDs) are semiconductor devices having P-Njunctions which, when appropriately connected to a power supply,generate light. The high efficiency of the light generation in the shortwavelength part of the light spectrum (e.g., ultra-violet and blue) hasprompted the use of LEDs in many applications, including simplecharacter or signal displays and more complex colored image displays.The use of LEDs for flashlights, brake lamps, signboards, etc, is wellknown. The current technique for the creation of colored light relies onthe UV or blue emission of the nitride-based LEDs to excite phosphorgranules embedded in an epoxy dome that, in turn, could generate greenor red light. To generate white light, the blue light of the LED ismixed with the green and red emissions of phosphor granules. Thistechnique, however, has low efficiency at the phosphor level and theepoxy encapsulant tends to degrade, thereby degrading the transparencydue the interaction with high energy blue light. The emergence of whiteLEDs as potential sources for illumination, expected to replaceconventional incandescent and fluorescent lighting, is based on thedemonstrated high efficiency in the short wavelength spectrum, on therelative long lifetime and reliability of the devices, and on theirsimple and versatile control, potentially allowing color adjustments,just to mention a few of LEDs' advantages. However, it has been foundthat the high conversion efficiency of nitride LEDs cannot be easilyextended toward wavelengths longer than blue, and this situation hasbecome known in the literature as “the green gap.”

Current LED operation is based on the fabrication of a multi-quantumwell structure (MQW) in the active volume of the device that issandwiched between n-type gallium nitride (n-GaN) and p-type galliumnitride (p-GaN) regions. The MQWs are also a succession of high bandgap/low band gap layers of nanometer size, typically GaN/InGaN, thatcreate within the quantum well intermediate energy levels between thatof the high and low band gap of the material system. The distribution ofthe intermediate energy levels defines the device color and depends onthe geometric parameters of the MQW structure as well as on the band gaplevels of the component materials. Green color, for example, extends inthe range of wavelengths from 495 nm to 570 nm. For the middle of thegreen domain range (˜532 nm), a nitride GaN/InGaN semiconductor alloysystem would have the lowest material gap of about or lower than 2.33eV. This band gap requires a fraction of at least 29% indium in thecomposition of InGaN.

To exhibit high quantum efficiency, the formation of carrierrecombination centers, such as misfit or threading dislocations, has tobe avoided. Misfit strain arises from the difference between thein-plane lattice parameters of the epilayers. A basic concept in theepitaxial growth is that for very thin layers it is energeticallyfavorable to accommodate the misfit strain elastically, while forthicker epilayers the strain is accommodated by introducing defects suchas misfit dislocations. For this reason, the alternating layers aregrown pseudomorphically, which requires the thickness of each of thealternating layers to remain below the limit at which the atomic bondsbreak and dislocations are formed. This limit is known as the criticalthickness. The difficulty in the epitaxial fabrication of such a MQWsystem relates to the elastic properties of the InGaN/GaN layer system.According to various models, such as the van der Merwe model (e.g.,Matthews, J. W. and Blakeslee, A. E., “Defects in epitaxialmultilayers: 1. misfit dislocations,” J. Crystal Growth, Vol. 27,December 1974, pp. 118-125, incorporated herein in its entirety), theforce balance model (e.g., Srinivasan, S., Geng, L., Liu, R., Ponce, F.A., Narukawa, Y., and Tanaka, S., “Slip systems and misfit dislocationsin InGaN epilayers,” Appl. Phys. Lett., Vol. 83, No. 25, December, 2003,pp. 5187-5189, incorporated herein in its entirety), or the energybalance model (e.g., Park, S.-E., O, B., and Lee, C.-R., “Strainrelaxation in InxGa1-xN epitaxial films grown coherently on GaN,” J.Crystal Growth, Vol. 249, No. 3-4, March 2003, pp. 455-460, and Lü, W.,Li, D. B., Li, C. R., and Zhang, Z., “Generation and behavior ofpure-edge threading misfit dislocations in InxGa1-xN/GaN multiplequantum wells,” J. Appl. Phys., Vol. 96, No. 9, November 2004, pp.5267-5270, incorporated herein in their entirety), the criticalthickness decreases with the increase of the indium fraction. The forcebalance and energy balance models both predict a critical thicknesssmaller than 2 nm for an Indium fraction of 30%, that decreases furtherwith the increase of the Indium fraction. Experimental data suggests acritical thickness of approximately 3 nm at a 20% Indium fraction, avalue that is lower than the approximate 6 nm value predicted by theaforementioned force balance or energy balance models. These resultssuggest that the fabrication of MQW LEDs having quantum wells withIndium fractions in the range of 30% or larger is associated with theincrease of the misfit and threading dislocation density—non-radiativedefects—and a sudden decrease of the overall radiative carrierrecombination efficiency, phenomenon known as the green gap.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cutaway side view illustrating a layerstructure according to an embodiment of the invention, along with banddiagrams showing properties of individual layers with and withoutexternal bias.

FIG. 2 shows a measured composition profile of a synthesized LEDstructure in accordance with an embodiment of the invention.

FIG. 3 shows a diagram of the emission intensity vs. wavelength for theLED structure shown in FIG. 2. The electroluminescence spectrum shows apeak with a wavelength of 540 nm (2.3 eV) in the green part of thevisible spectrum.

FIG. 4 shows a schematic cutaway side view illustrating a layerstructure according to an embodiment of the invention, along with banddiagrams showing properties of individual layers with and withoutexternal bias.

FIG. 5 shows a diagram of electroluminescence intensity vs. photonwavelength for a graded p-n junction LED under increasing forward bias.

DETAILED DESCRIPTION OF INVENTION

One or more embodiments of the present disclosure are directed to alight emitting device (LED) capable of emitting light over a widespectrum, from ultra-violet to near-infrared, without the use ofphosphor materials, based on three recent advancements: (1) progress inthe growth of group III-nitrides on Si substrates; (2) successfulformation of p/n junctions in partially phase-separated InGaN; and (3)improved ability to grow compositionally graded n-type and p-type layersof InGaN, with a composition range from 0% to 40% In. Variousembodiments are described herein using the case of InGaN alloys, but thepresent teachings are also applicable to InAlN ternary or InGaAlNquaternary alloys. Further, while various embodiments are describedherein using the case of group III-nitrides formed on Si substrates, thepresent teachings could in principle be formed on other substrates, suchas sapphire or SiC, with properly modified growth conditions.

In a first embodiment, the basic structure of the new light emittingdevice consists of a heavily doped (e.g., doped at 5×1018-2×1019 cm-3)p-type Si (111) substrate followed by a buffer layer, as shown inFIG. 1. In FIG. 1, section a shows the layer structure of the lightemitting from the device, section b shows a band diagram with no biasapplied, and section c shows a band diagram with external bias, V_(ext).While the buffer layer in the illustrated embodiment consists of a thinAlN nucleation layer, any other suitable nucleation layer may beutilized. The buffer layer is followed by an n-type GaN layer. In oneembodiment, the n-type GaN layer is doped at 10¹⁸-10¹⁹ cm⁻³. The GaNlayer is followed by an n-type InGaN layer graded from GaN to a targetInGaN composition having a desired percentage of In. In one embodiment,the GaN layer is followed by an n-type InGaN layer graded from GaN to atarget InGaN composition having 40% In on average. However, it should benoted that the target composition depends on the desired LED colorrange. The graded n-type InGaN layer is followed by an n-type InGaNlayer and a p-type InGaN layer of the target composition (i.e., thetarget InGaN composition having a desired percentage of In). The p/njunction is formed in the InGaN of the target composition or in thegraded n-type region. The alloy composition and/or the thickness of theInGaN p/n junction is adjusted to optimize the intensity and to selectthe wavelength of the emitted light. For example, the parameters of thetarget composition of the InGaN p/n junction can be adjusted to allowthe device to be tuned to produce Green and longer wavelength LEDs orother multi-color and/or white light spectrums. The p/n InGaN layers ofthe constant target composition are followed by a p-type InGaN layergraded from the target InGaN composition to GaN (i.e., the final gradingis absent In).

Finally the structure is capped with a layer of p-type GaN. An ohmiccontact (not shown) for injecting current is formed on the back of thep-Si substrate using a metal deposition. A top ohmic contact is formedon the top surface of the p-type GaN layer. This top contact to thep-type GaN can be made using a semi-transparent NiAl layer, a metallicgrid or a transparent conducting oxide (TCO). Details as to theformation of ohmic contacts can be found in PCT Patent Application No.PCT/US2008/004572 entitled “Low Resistance Tunnel Junctions for HighEfficiency Tandem Solar Cells” filed Apr. 9, 2008, and in U.S.Provisional Patent Application No. 60/910,734 filed Apr. 9, 2007, theentire disclosures of which are incorporated herein by reference. Inthese disclosures, the inventors teach how p-type Si forms a lowresistance ohmic contact with n-type InGaN. In the context of thepresent invention, this will facilitate the electrical contact from thesubstrate to the active device.

In one or more embodiments, the group III-nitride layers (or otherlayers) are deposited using molecular beam epitaxy (MBE) techniques, butit is understood that the various layers can also be deposited usingmetal-organic chemical vapor deposition (MOCVD), hydride vapor phaseepitaxy (HVPE), remote plasma chemical vapor deposition (RPCVD), or anyother appropriate deposition method.

The energy band diagram of the device structure is schematically shownin FIG. 1 at section b. The graded region on the n-type side of thedevice structure confines the holes whereas the graded region on the pside confines the electrons.

InGaN undergoes phase separation into the regions of larger and smallerIn content. The phase separation is especially severe at high Incontent, e.g., greater than 30% In content. The origin of thisphenomenon is not well established. However, there are indications thatit could be attributed to the higher chemical binding energy of GaNcompared with InN molecule. Consequently, with higher Ga content, e.g.,greater than 60% Ga, larger band gap regions will be formed at growthinitiation regions such as grain boundaries and dislocations, whereaswith large In content, e.g., greater than 30% In, small band gap regionswill be removed away from such internal surfaces. This also leads tospatial variations of the carrier lifetime as the regions close to thegrain boundaries and/or dislocations have larger densities ofnon-radiative recombination centers. Clusters with larger In content areexemplary indicated as a few wells in the band diagrams in FIG. 1 atsections b and c, and in FIG. 4 at sections b and c.

When, as shown in FIG. 1 at section c, the device structure is forwardbiased, the electrons (e) and holes (h) are injected into the InGaNregion. They tend to agglomerate in the high In, low band gap regions.The observed electroluminescence (EL) energy is expected to be lowerthan that predicted by the average composition. Also, the EL should bemore efficient because of the lower density of non-radiative centers inthe In-rich region.

The present inventors have grown and tested several device structures asshown in FIG. 1 at section a. The testing included a compositionvariation measured by Rutherford Backscattering Spectroscopy (RBS), withthe results of such testing shown in FIG. 2. In particular, FIG. 2 showsa composition profile of an LED structure in accordance with anembodiment of the invention, as determined by RBS. In the insert,maximum average indium fraction determined in the InGaN region is ˜8%.The p/n junction in the InGaN layer was fabricated using Mg doping. Theaverage In composition of the InGaN junction region was 8% (i.e., thetarget InGaN composition possessed 8% In). However, EL measurements ofthis device structure have shown, as illustrated in FIG. 3, a strongpeak at 2.3 eV in the green range of the visible spectrum (540 nm).

This emission energy corresponds to much larger In content of 30%. Thelow energy EL indicates phase separation and formation of clusters withIn composition of 30%, which is larger than the average In content of8%. In accordance with one or more embodiments, a key advantage here isthat the p/n junction is formed in the InGaN target layer before the topgraded layer. The rectifying properties are determined by the low Incomposition (majority) phase, whereas emission properties are mainlydetermined by high In composition (minority) phase (i.e. a large bandgap diode is used to produce lower energy photons).

For structures grown under different conditions, the phase separationextends into the InGaN graded regions, toward the lower indium fractionInGaN, as is the case in the embodiment illustrated in FIG. 4. In FIG.4, section a shows the layer structure of a second embodiment of thedevice, and sections b and c show band diagrams with no external biasapplied and with applied external bias applied, respectively. In thiscase, when the device structure is forward biased, at low bias values,e.g., less than 3V, the electron-hole recombination will take placepredominantly across the low band gap InGaN clusters generating lightwith long wavelengths. As the forward bias is increased, the carrierswill be injected into larger band gap regions, shifting the emissionfrom long to shorter wavelength. This operation of the graded p-njunction LED is demonstrated by the electroluminescence measurementsunder increasing bias presented in FIG. 5. This demonstrates thetunability of the improved LEDs described herein to multi-color andwhite light spectrums.

The above-described embodiments are not limited to the specific alloycompositions described. Judicious choice of the average composition andthe growth conditions will allow fabrication of devices emitting lightfrom red to blue providing basic components for light generationcovering the full visible spectrum.

In accordance with one or more embodiments of the LED described herein,a long wavelength LED device is provided that utilizes low cost siliconwafer substrates. Low cost Green and longer wavelength LEDs have beensought after by both science and industry for an extensive period oftime because they would fill a high-value gap in the rapidly growingglobal LED market for lighting and illumination where energy efficiencyand miniaturization is paramount to the global renewable energymomentum. Green or longer wavelength nitride based LEDs are verychallenging to fabricate compared to UV and Blue LEDs due to decreasingquantum efficiencies and have remained a tough milestone for the LEDindustry. In accordance with one or more embodiments of the LEDsdescribed herein, efficient long wavelength LEDs are provided that allowfor the achievement of essential milestones in the roadmap for SolidState Lighting (SSL), LED backlighting and next generation displaytechnology.

One application of the present invention is in photovoltaic devices. Forexample, the present invention may be used in combination withphotovoltaic devices having three-dimensional charge separation andcollection, such as those taught in U.S. patent application Ser. No.13/312,780 entitled “Photovoltaic Device With Three Dimensional ChargeSeparation And Collection” filed Dec. 6, 2011, the entire disclosure ofwhich is incorporated herein by reference. In the referencedapplication, the inventors teach using a graded InGaN p/n junction for acharge separation in a photovoltaic device.

The above embodiments and preferences are illustrative of the presentinvention. It is neither necessary, nor intended for this patent tooutline or define every possible combination or embodiment. The inventorhas disclosed sufficient information to permit one skilled in the art topractice at least one embodiment of the invention. The above descriptionand drawings are merely illustrative of the present invention and thatchanges in components, structure and procedure are possible withoutdeparting from the scope of the present invention as defined in thefollowing claims. For example, elements and/or steps described aboveand/or in the following claims in a particular order may be practiced ina different order without departing from the invention. Thus, while theinvention has been particularly shown and described with reference toembodiments thereof, it will be understood by those skilled in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope of the invention.

The invention claimed is:
 1. A light emitting device capable of emittinglight, comprising: a substrate; a plurality of n-type layers, eachcomprising a group III nitride alloy, wherein said plurality of n-typelayers comprise at least one layer of n-type graded group III nitride; aplurality of p-type layers, each comprising a group III nitride alloy,wherein said plurality of p-type layers comprise at least one layer ofp-type graded group III nitride; a first ohmic contact for injectingcurrent formed on the substrate; and a second ohmic contact formed on asurface of at least one of the p-type layers.
 2. The light emittingdevice according to claim 1, wherein an alloy composition and/or thethickness of an InGaN p/n junction is adjusted to optimize the intensityand to select the wavelength of the emitted light.
 3. The light emittingdevice according to claim 1, wherein the Group III Nitride alloycomprises InGaN.
 4. The light emitting device according to claim 1,wherein the Group III Nitride alloy comprises an InAlN ternary alloy. 5.The light emitting device according to claim 1, wherein the Group IIINitride alloy comprises an InGaAlN quaternary alloy.
 6. The lightemitting device according to claim 1, wherein the substrate comprisesSilicon.
 7. The light emitting device according to claim 6, wherein thesubstrate comprises a doped P-type Silicon substrate.
 8. The lightemitting device according to claim 1, wherein the substrate comprisessapphire.
 9. The light emitting device according to claim 1, wherein thesubstrate comprises Silicon Carbide.
 10. The light emitting deviceaccording to claim 1, wherein said wide spectrum is from ultra-violet tonear-infrared.
 11. The light emitting device according to claim 1,wherein a p/n junction is formed in InGaN of a target composition. 12.The light emitting device according to claim 1, wherein a p/n junctionis formed in a compositionally graded n-type region.
 13. A lightemitting device based on group III-nitride semiconductors grown on aSilicon substrate capable of emitting light over a wide spectrum, fromultra-violet to near-infrared, without the use of phosphor materials,said light-emitting device comprising one or more p-type compositionallygraded group III nitride layers and one or more n-type compositionallygraded group III nitride layers.
 14. A light emitting device comprising:a heavily doped p-type Si substrate; a buffer layer formed on the p-typeSi substrate; an n-type GaN layer formed on the buffer layer; acompositionally graded n-type InGaN layer formed on n-type GaN layer andgraded from GaN to a target InGaN composition having a desiredpercentage of In; an n-type InGaN layer formed on the compositionallygraded n-type InGaN layer and a p-type InGaN layer formed on the n-typeInGaN layer, wherein the n-type and p-type InGaN layers comprise thetarget InGaN composition and form the p/n junction, further wherein thethickness of the target composition of the InGaN p/n junction isadjusted to optimize the intensity of the LED and to select thewavelength of the emitted light; a graded p-type InGaN layer formed onp-type InGaN layer and graded from the target InGaN composition to GaN;a p-type GaN layer formed on the graded p-type InGaN layer; an ohmiccontact for injecting current formed on the back of the p-type Sisubstrate; and a top ohmic contact formed on a top surface of the p-typeGaN layer.
 15. The light emitting device according to claim 14, whereinthe buffer layer comprises a nucleation layer.
 16. The light emittingdevice according to claim 14, wherein the nucleation layer comprises athin AlN nucleation layer.